In carrying out Doppler color flow mapping (CFM) by the conventional ultrasonic diagnostic apparatus, the pitch p of oscillatory elements and the wavelength .lambda. of an ultrasonic sounding wave are determined so as to meet an inequality, p&lt;.lambda./2 so that only a main lobe exists in the visual field, i.e., so that no grating lobe exists in the visual field, and the visual field is scanned by the main lobe.
The grating lobe is prevented from existing in the visual field to prevent formation of a virtual image by the grating lobe.
Although not directly related to the present invention, because the grating lobe is different from the side lobe, a technique of preventing a virtual image attributable to the side lobe is disclosed in Japanese Patent Laid-open (Kokai) No. 61-11030.
In the Doppler CFM by the ultrasonic diagnostic apparatus, the real-time performance can be enhanced by increasing the frame rate F, the visual field can be enlarged by increasing the number N of sound rays, and the S/N ratio can be improved by increasing the pulse repetition frequency P.
However, since the relation between the frame rate F, the number N of sound rays, the pulse repetition frequency (the number of successive pulses to cause the phase variation of one sound ray) P, the maximum detection depth Dmax and the sound velocity c is expressed by: F.times.N.times.P.times.2D.sub.max /c&lt;1, the increase of the frame rate F, the increase of the number N of sound rays, and the increase of the pulse repetition frequency P contradict each other.
Accordingly, the enhancement of the real-time performance, the expansion of the visual field or the improvement of the S/N ratio, entails inevitably the sacrifice of the visual field or the S/N ratio, the real-time performance or the S/N ratio, or the real time performance or the visual field.